It appears to depend on firmware or kernel but for recent kernels (recent 6.1 or 6.2) activelow is no longer necessary [1]. It seems to work on CM4.
PS: my bad I disabled the LEDs via /sys not /proc previously:
> # for i in /sys/class/leds/*; do echo 0 > "$i/brightness"; done
[1] https://github.com/raspberrypi/firmware/issues/1742
The required lines were different between the Pi 2 and Pi 4 I was using at the time, and the documentation was not updated (or I'd found old docs).
An exercise in trial and error.
It appears to depend on firmware or kernel but for recent kernels (recent 6.1 or 6.2) activelow is no longer necessary [1]. It seems to work on CM4.
PS: my bad I disabled the LEDs via /sys not /proc previously:
> # for i in /sys/class/leds/*; do echo 0 > "$i/brightness"; done
[1] https://github.com/raspberrypi/firmware/issues/1742